1. Assistant Professor, Federal Univ. of Rio Grande do Sul, 90035-190 Porto Alegre, Rio Grande do Sul, Brazil.
2. Reader, Dept. of Civil and Environmental Engineering, Imperial College London, Imperial College Road, SW7 2AZ, London, UK
3. Associate Professor, Dept. of Civil Engineering, Federal Univ. of Rio Grande do Sul, Ave. Osvaldo Aranha, 99, 3. Andar, 90035-190 Porto Alegre, Rio Grande do Sul, Brazil, (corresponding author)